Thanks to JohnoBoy, the solution to Sony's ridiculous money region lock is twofold:
masking your IP to make it look like it is coming from US. I did it with VPN, which is the most secured way to do it since your are accessing your private PSN account that also contains your credit card information. I don't recommend proxy servers because some free proxy servers keep log of people's information and sell them to third party. It could help others to compromise your PSN account and your credit card.
Add funds through SEN website under "Account -> Wallet". This new online feature is probably the most useful thing that Sony did to their dinosaur PSN website.
An alternative is to buy PSN cards codes from Amazon* or Maximus cards. They both email you the code instantly and thankfully Sony don't region lock the PSN card.
*NOTE: Amazon only require you to provide a US billing address, even with a non-US card. This has been tested by JohnoBoy. The plus side with Amazon is that they don't charge extra on top of the face value of the PSN card. They actually give you 1 cent off. Maximus cards on the other hand charges $2.49 ~ $8.59 extra for a single card, depending on the face value of the card you are buying.